It’s not really a “need” but it’s to do with power dynamics, some people naturally like to be in control or give control to someone else, and then there are people who like to take control from someone who would usually be in control, it’s an exchange of power and dominance but equally being submissive is another form of power. It depends on the relationship, the dynamics and how much power is actually being exchanged beyond who actually puts it in or not
But It's always the same in yaoi. Omegas are bottoms. Once someone switched they switched forever. He not only changed his preferences in bed but he also started acting a lot sweeter and gentler... His looks are also softer. For people who wanted to see some switching couple or omega on top this is a letdown.

I dont get why there is this need to turn tops into bottoms. I mean sure,he likes being a bottome but he also likes being on top. But it feels weird to showcase it so much in these mangas. Just because he is an omega doesnt mean he needs to be an owo little obedient baby that moans like a girl.
Ah idk,its just weird to see that trope so much imo